1. Field of the Invention
The present invention relates to anode (negative electrode) active materials and methods of preparing the same. More particularly, the invention relates to anode active materials comprising carbonaceous materials.
2. Description of the Related Art
Non-aqueous electrolyte secondary batteries using lithium compounds as anodes have been intensively researched and developed in an effort to obtain high voltages and high energy densities. Specifically, in the early stages of the research and development metallic lithium was intensively researched because of its high battery capacity, yielding much attention for lithium as the most prominent anode material. However, when metallic lithium is used as an anode material, a large amount of lithium is deposited on the surface of the anode in the form of dendrites. This may degrade charging and discharging efficiencies or cause internal-shorts between the anode and the cathode (positive electrode). Further, lithium is very sensitive to heat and impact and is prone to explosion due to its instability (i.e., high reactivity). This has delayed commercialization. In order to address these problems, carbonaceous materials have been proposed for use as anode materials. Carbonaceous anodes perform redox reactions enabling lithium ions in the electrolytic solution to intercalate and deintercalate into and out of the carbonaceous anode. The carbonaceous material of the anode does not include lithium metal and may have a crystal lattice structure during charge and discharge cycles, which is referred to as a “rocking chair type” anode.
Carbonaceous anodes have greatly contributed to the widespread use of lithium batteries by overcoming various disadvantages associated with metallic lithium. However, as electronic devices become smaller and lighter, and use of portable electronic instruments increases, development of lithium secondary batteries having higher capacities becomes a major focal point. Lithium batteries using carbonaceous anodes basically have low battery capacities because of the porosity of the carbonaceous anode. For example, when made into a LiC6 alloy by reaction with lithium ions, graphite (an ultra-high crystalline material) has a theoretical capacity of about 372 mAh/g. This is only about 10% that of metallic lithium, i.e., 3860 mAh/g. Thus, in spite of the many problems with conventional metallic lithium anodes, studies on the improvement of battery capacities of batteries using metallic lithium anode materials are still actively being conducted.
Lithium-containing alloys such as lithium-aluminum, lithium-lead, lithium-tin, or lithium-silicon have higher electrical capacities than carbonaceous materials. However, when such an alloy of two or more metals or a single metal is used, formation of lithium dendrites may cause several problems, e.g., short-circuiting of the battery.
Accordingly, various studies on solutions to these problems and on enhancing the electric capacities of batteries by mixing the lithium-containing alloys with carbonaceous materials have been undertaken. However, differences in volumetric expansion between metallic materials and carbonaceous materials may cause the metallic material to react with the electrolyte. During charge cycles, lithium ions used as the anode material are inserted into the anode. When this happens, the overall volume of the anode expands, leading to a compact anode structure. During discharge cycles, lithium is deintercalated from the anode to an ionic state, resulting in a reduction in the volume of the anode material. Since the metallic material and the carbonaceous material have different expansion coefficients, empty spaces may be created in the anode structure when the anode material shrinks. This causes electrical isolation in the openings formed in the empty spaces, thereby disabling smooth mobility of electrons and deteriorating the lifespan of the battery.